The present invention relates to sheet and document handling devices, in particular to devices which assist the movement of sheet, or web, as it is being drawn into a document finishing machine or like device.
When sheet, in particular, perforated edge fanfold paper sheet, also referred to herein as web, is drawn from a supply such as a stack into various types of commercial document finishing devices, it is inherent that the motion of the sheet is alternately ceased and then resumed, as the device does certain operations. For instance, if the finishing machine is converting the sheet into pages of forms, and accumulating them, there will be an unsteady rate of sheet movement. It is an observed problem that paper sheet will tend to tear under such situations; obviously, it is due to the tensile strains attending rapid acceleration of sheet.
The tendency for tearing, or even erratic web motion without tearing, limits the rate at which certain finishing machines can process sheet. Tearing can require repeated operator intervention and inferior production. There are numerous installed commercial machines which exhibit such limitations. Thus, there is a need for some kind of device which can be placed upstream of a finishing machine or other processor, to smooth out, or buffer, the sheet motion, and to thereby lessen the tendency of the sheet to tear and to allow higher average sheet speeds and production. Things that have been tried to improve operation. For instance, a dead weight roller or other object has been hung on the sheet to form a loop on the flow path between the supply and the finishing machine. Fan blown air has also been directed at the sheet running along the flow path.
There are prior art devices which are designed to assist the feeding of web. They have been used when the pulling capacity of a constant input speed finishing machine has insufficient power to draw the sheet into the machine. A typical device of such type is comprised of three fixed-position driven rollers. The sheet follows a serpentine path. The speed of the rollers is varied so that the feed rate corresponds with the speed at which the finishing machine demands sheet. Such devices are not known to have been used, nor has the work on the present invention shown them, to be suitable for solving the problem which is described, where the finishing machine demands web at a variable speed and in a cycle which includes stopping, and where the cycle is repeated at high frequency.
An object of the invention is to lessen the forces on a sheet or web which is subjected to high acceleration and deceleration, to decrease any tendency for the sheet to tear when the sheet is being drawn into a finishing machine or other processor. A further object of the invention is to provide an improved means for assisting the movement of sheet along a path, where the amount of force imparted to the sheet is readily adjustable.
In accordance with the invention, apparatus for controlling the movement of sheet downstream along a flow path, toward a finishing machine device which pulls the sheet downstream from a source or supply with frequent change in velocity, comprises an assist unit, for urging the sheet downstream; and a dancer unit, preferably a resiliently biased dancer unit, positioned downstream of the assist unit, for dynamically changing the length of the flow path between the sheet source and the device. The combination of assist unit and dancer unit change the velocity vs. time cycle to which sheet is subjected at points upstream of the finishing machine entrance, compared to the cycle at the finishing machine, to lower the acceleration of the sheet and the tension in the sheet.
In one aspect of the invention, the tension ratio of the assist unit is less than 6 to 1, preferably less than 3 to 1, where tension ratio is the ratio of the tension in sheet at the input side of the assist unit divided by tension in sheet at the output side of the assist unit, measured when the assist unit is acting on a piece of sheet being restrained in place. In another aspect of the invention, the assist unit has at least two drive rollers and the total angle of wrap of sheet about all the drive rollers is no more than 2 xcfx80 radians, preferably 3 xcfx80/2. In another aspect of the invention, the velocity of sheet through the assist unit is less than the rotational surface speed of the at least one drive roller. The invention may include a drag unit located upstream of the assist unit. However, in some systems the inherent drag on the sheet may not demand a separate drag unit.
In a preferred embodiment, the assist unit is comprised of only two drive rollers. The sheet flow path through the assist unit has an S-shape, as the sheet wraps around a portion of the exterior of each roller. The orientation of the drive roller pair is changeable, to a desired fixed position, to effect a change in shape of the S path through the assist unit. Thus, the angle of wrap of sheet within the assist unit and the tension ratio of the assist unit can be changed to a desired predetermined level. Preferably, one roller axis stays at a fixed position and the second roller is journaled in a pivotable mounting block, so the second roller moves with planetary motion about the first. Preferably, the assist unit runs at constant speed.
In a preferred embodiment, the dancer unit is comprised of a dancer roller assembly which translates vertically in space. The roller assembly has quite a low mass and is spring biased in a direction which increases the flow path length. An exemplary low mass dancer roller is constructed of thin wall plastic tubing. The weight of the roller assembly ought to be much less than 4 gF/aweb, where F is the maximum tension the sheet can sustain and aweb is the maximum acceleration of the sheet at the inlet of the finishing machine and g is the acceleration of gravity. In proper use, the combination of low mass roller and spring bias keeps the roller in close proximity to the sheet as the flow path dynamically changes, thus avoiding roller impulse forces on the sheet which might tear the sheet when the roller substantially separates from contact with the sheet. Preferably, the dancer roller assembly translates along a path defined by vertical rails and the sheet follows a narrow V-shape flow path around the dancer roller.
In the method of the invention the motion of sheet is affected at sequential points along the flow path between a supply of sheet and the finishing machine which pulls the sheet with frequent change in velocity, including periodic stopping of the sheet, thereby creating a certain acceleration and tension in the sheet at the entrance to the finishing machine. A drag force is applied at a first point, a force which urges the sheet downstream is applied at a second downstream point, and a resilient force is applied to the sheet at a third further downstream point. The flow path is changed in length inversely with change in velocity of the sheet at the finishing machine. The tension in the sheet as it enters the finishing machine is reduced.
In operation, the invention reduces the stress which is generated in the sheet as a result of the action of the finishing machine. It thus reduces any tendency for tearing, and it improves the operation of most finishing machines, making them capable of high speed operation.
The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings.